A common requirement for Dental CAD/CAM systems is the acquisition of a true three-dimensional representation of the patient situation—that is, the shape of the patient's existing teeth, gums and palette—with the appropriate degree of accuracy for the prosthetic that is to be designed. Three-dimensional scanning systems sold to dental labs most often digitize the “stone” model, made by plaster casting the patient impression made by the dentist. In some cases, the patient impression is digitized directly. Three-dimensional scanning systems sold directly to dentists often employ “intra-oral scanning” techniques where the digitizer is inserted directly into the patient's mouth.
Different range sensing techniques have been used to engineer dental lab scanners and intra-oral scanners, including: triangulation, phase-shift reconstruction, conoscopic holography, confocal microscopy, and time of flight. Most commercial dental lab scanners use triangulation and phase-shift reconstruction. These both work through projecting structured light onto the object to be digitized, capturing an image (or images) with a frame grabber, and then reconstructing the image on the computer to produce Point Cloud (X, Y, Z) data relative to the point of view of the image capture device(s).
Conoscopic holography is based on crystal optics and interference patterns generated by interacting polarized light rays. The NOBEL BIOCARE™ dental lab scanner uses this technique.
Confocal microscopy uses an image capture system with a very narrow field of focus and then varies the focal plane in a known sequence. Several intra-oral scanners, such as the ITERO™ by CADENT™, use the confocal principle to construct Point Cloud data.
Time of flight systems direct light (usually from a laser source) against an object and measure the amount of time to detect the reflection. Since the speed of light c is constant, the distance to the object may be calculated. Because the precision of the time measurement is limited, and the accuracy requirements for dental scanners is relatively high (in the range of 10-30 microns), time of flight has not yet been used for dental scanners.
Three-dimensional scanning is largely done with hardware and software that is dedicated to scanning, rather than with general purpose hardware and software. Three-dimensional scanning devices are generally classified according to the underlying technology, such as white light, non-white light, point, line, and phase change. Three-dimensional scanning devices may operate in one of three modes: fully automatic, semi-automatic, and manual.
One of the central problems in capturing scan data for analysis in creating a three-dimensional model is that of controlling camera position and orientation relative to the object being scanned. Given a fixed camera focal length based on the lens configuration, if the camera is at the optimal focal distance from the object, then mathematically precise scan data may be extracted from the images. By combining a sufficient number of such images, and by knowing the position of the object and the camera locations and orientations from which the images were collected, a high quality three-dimensional reconstruction may be created. Failing to control camera position and orientation, however, may lead to ambiguity in the collected data, thereby rendering the three-dimensional reconstruction an approximation with unknown accuracy.
There is a need for improved methods, systems, and apparatus for scanning an object to produce a virtual three-dimensional representation of the object.